With mixtures of hydrogen and oxygen gas it is necessary constantly to check how high the content of foreign gas or minority gas is, i.e. oxygen in the carrier gas hydrogen or hydrogen in the carrier gas oxygen. If there is too high a concentration of the foreign gas, there is a risk of explosion. The lower explosion limit for mixtures of hydrogen and oxygen is a foreign gas concentration of around 4%. Said foreign gas concentration applies both for hydrogen as a foreign gas in oxygen and for oxygen as a foreign gas in hydrogen.
Electrolysis of water produces hydrogen at the hydrogen cathode and oxygen at the oxygen cathode. However, the hydrogen gas produced always contains a slight amount of oxygen and the oxygen gas produced has a hydrogen content. In the latter case in particular, the foreign gas concentration has to be monitored to ensure that it is a sufficient safety margin below the lower explosion limit in order to avoid a risk of explosion.